The invention relates to functions and methods for interworking frame relay with asynchronous transfer mode and more particularly to methods and functions which provide call setup capabilities.
In FR (frame relay), data is sent in packets which have a two byte header and have a variable size data section (payload) which ranges in size from two to 8187 bytes. FR is connection oriented, with each packet including a connection identifier, the packets of a given connection constituting a logical frame relay connection. Each FR logical connection is identified by a DLCI (data link connection identifier). Currently, DLCIs range from 0 to 1023 with DLCI-16 through DLCI-1007 allocated for data connections. Typically, DLCI-0 is reserved to function as a signalling connection. This connection is used to set up new logical connections for example.
Between two FR switching elements there is a capability of assigning a transfer priority to each connection typically in the range of 0 to 15. FR messages include C-plane messages which are used for signalling, and U-plane messages which are used for data, the C-plane messages requiring a higher priority than U-plane messages. Between two FR switching elements high priority LMI (local management interface) C-plane messages are used to implement a handshaking protocol. This high priority assigned to LMI traffic thereby ensures that essential signalling information can get through the network notwithstanding congestion which may otherwise exist.
In ATM, data is sent in cells which have a fixed size 53 bytes including a five byte header. ATM is also connection oriented with ATM cells being carried in VCCs (virtual channel connections). One VCC represents one connection in much the same way that one DLCI represents one FR connection. Each VCC has a QoS (quality of service). The QoS""s presently defined include CBR (constant bit rate) typically used for voice, VBR-rt (variable bit ratexe2x80x94real-time) used for real-time sensitive services, VBRxe2x80x94non-real-time used for services which are not real-time sensitive, UBR (unspecified bit rate) used for low priority traffic, and ABR (available bit rate) which has recently been introduced. The QoS for a VCC is fixed across all DLCIs carried over the VCC.
There needs to be the capability to connect FR devices to ATM devices and vice versa, and this capability is usually implemented by some kind of IWF (interworking function). At an IWF, this has consisted of some sort of mapping between VCCs, and incoming/outgoing logical frame relay connections (DLCIs).
Two existing FR-ATM interworking solutions adhere to two schemes of multiplexing presented in ITU-T recommendation I.555. There is also a solution proposed by FRF.5 which builds upon that of ITU-T I.555. In all of these solutions, control/signalling information is carried with no distinction from data across each ATM connection. In other words, the ATM network gives the same priority to signalling messages as to the data itself.
Historically, FR has been thought of as being applicable to non-real time applications, and as being most suitably carried by ATM VBR-nrt. Since signalling packets can be expected to satisfy VBR-nrt behaviour and the data can also be modelled as VBR-nrt, the above problem of giving the same priority to signalling as data has not been significant. However, now real-time applications for FR exist, for example voice over FR, in which case it would be useful to be able to carry FR traffic with different QoS and still be able to reliably transmit control/signalling information. The existing solutions do not permit this. If for example, the ATM UBR service is used for the data and control it is likely that some packets will be lost. While this may not be serious if a regular data packet gets lost, it is a problem if control and signalling information is not getting through reliably.
The existing interworking solutions do not provide any way of setting up FR SVCs which cross an ATM network.
It is an object of the invention to obviate or mitigate one or more of the above identified disadvantages.
A system and method for interworking between FR (frame relay) and ATM (asynchronous transfer mode) using permanent virtual circuits and switched virtual circuits are provided. An ATM VCC between two interworking functions is dedicated to FR LMI signalling traffic, while the FR data traffic is carried by ATM VCCs which do not carry any FR LMI signalling traffic. The FR data traffic of a given FR DLCI is carried by an ATM VCC having a service category and bandwidth parameters which reflect the transfer priority and bandwidth needs of the particular FR DLCI. This allows a high priority to be assigned to all LMI traffic carried over the ATM network independent of the priority assigned to the FR data traffic. FR SVC (switched virtual circuit) call setup messages requesting a new SVC connection are carried over the dedicated channel. An attempt is first made to allocate space on an existing ATM VCC with appropriate service characteristics to handle the connection. If one is not available, then a new ATM VCC with the required service characteristics is established.
Advantageously, LMI and FR SVC setup signalling can be given a higher priority on the ATM network than normal data traffic, thereby ensuring it is reliable. Furthermore, where in previous systems, FR SVC signalling has stopped at the FR-ATM boundary, the invention provides for methods and systems for propagating the FR SVC signalling across the ATM network to a far end FR device.